Cygwin64 support

I wanted to use ocaml, shared libraries enabled, in x86_64 Cygwin.
So I tryed to figure out how good the flexdll development version is doing.
I've found several minor problems and managed to fix those myself.
Could you please take a look and merge these changes?

Now I'm thinking of generating "JMP *__imp_XXX" thunks
to avoid "target is too far" error.